This invention relates to an improved method of manufacturing slabstock polymeric foam, especially but not exclusively polyurethane foam, and the invention also relates to an apparatus for use in the production of foam blocks by the improved method.
It is well known and common current practice to make polyurethane foam blocks by a continuous process involving the essential steps of laying down a mixture of foam reactants onto the bottom of a channel-shaped conveyor, which serves as a moulding trough of open-topped rectangular cross-section, allowing the mixture to foam and expand as it progresses along the channel-shaped conveyor, and when the expanded foam has cured, cutting the slabstock foam into blocks. It is usual for the bottom and sides of the conveyor to be lined with foils of paper and/or plastics which advance with the foam and prevent it from sticking to the conveyor walls. When the foam expands, the resulting foam bun tends to take on a crowned or domed shape at the top, as seen in cross-section. At least in part, this crowning effect is caused by the expansion of the foam being resisted by the foam being in frictional contact with the lining foils or walls at the sides of the conveyor. The convex shape at the top of the foam bun is highly undesirable since it reduces the useful volume of foam obtained and hence detracts from the economy of the manufacturing process.
Various proposals have been made for preventing the formation of the crowned top. For example, it has been suggested to raise the foils lining the side walls during the expansion in attempt to eliminate the frictional resistance. Another idea is to apply a top cover sheet, e.g. of paper, to the upper surface of the foam mixture soon after it is laid down on the conveyor, and as the foam rises, pressure is applied to the top surface through the top cover sheet to constrain the expanding foam and achieve a flat top transverse to the direction of conveyor movement. In GB 1487848 (Planiblock) for instance there is described an apparatus in which a series of rollers or runners (plates) carried on pivoted arms are arranged to bear down on the expanding foam. According to another proposal described in GB 1392859 (Hennecke), a levelling device in the form of a slatted grating or grid is arranged to float on the expanding foam mixture. The Hennecke system has been used commercially and has enjoyed a certain degree of commercial success. Applying the top cover sheet has an additional benefit in that it reduces the tendency for the foam to form a skin on the top surface due to the foam collapsing at this surface. However, this benefit is counteracted to some extent by the application of substantial pressure to the top surface by the weight of the levelling device, and as a consequence the foam has to rise against this pressure, and a skin of significant thickness is still produced in practice. A further drawback of applying substantial pressure to the top surface during foam expansion is that it can increase the variation in density over the height of the foam block, and especially in the upper layers, which means that some of the foam can fall outside specification requirements.
Another technique which has proved very successful in producing flat-topped foam slabs is described in GB 1354341 (Unifoam). According to this technique the foam reactants are delivered into a vessel or trough which extends transversely to the direction of conveyor advancement and which has an overflow outlet over which the mixture spills onto a downwardly inclined ramp or fall plate extending to the conveyor itself. The fall plate angle is adjusted so that, in essence, the foam expands downwardly and the top surface is maintained substantially level throughout the foam expansion process. This process is exploited commercially under the name "Maxfoam" and has been in successful operation for several years. The Maxfoam process is very effective in producing flat-topped blocks, whereby the conversion of the blocks is very economic. Nonetheless, it does have one drawback which hitherto has proved insurmountable. The problem is that there is a relatively thick skin formed on the top surface of the foam. The disadvantages of a thick skin are several fold. As much as approximately 10% by weight of the foam bun material can be in the skin layers which must be cut away from the usable foam, and of this figure typically around 4-5% by weight can be in the top skin. If the skin thickness could be reduced, it would mean a saving in raw materials consumed and a greater percentage of material would be converted into useful foam, and hence the economy of the process would be enhanced. It is usual for the skin layers to be sold by foam manufacturers as a lower grade material which, for example, can be chopped or granulated into small pieces and rebonded together to form certain foam products such as carpet underlay. If the skin is too thick, however, it is not even acceptable for such processing and it must be discarded or burnt, which is obviously uneconomic as well as being undesirable from an environmental viewpoint. The reason that a relatively thick top skin is formed in the Maxfoam process is due to the foam collapsing at the upper surface and the reaction between toluene diisocyanate (TDI) which is one of the raw materials used in the foaming process, and the atmospheric moisture.
It has long been recognised and appreciated that the skin thickness would be diminished if a top cover sheet could be applied over the surface of the foam mixture during its expansion to define a moulding surface to which the foam would adhere by interfacial surface tension to reduce the tendency for the foam to collapse back on itself. This phenomenon is well known from polyurethane foam moulding techniques in which complete articles, e.g., cushions are formed within closed moulds. In spite of many attempts, prior to the present invention it has proved beyond skilled workers to apply successfully a top cover sheet over the reacting foam mixture in the Maxfoam process. The reason the attempts have failed is explained as follows. The foam reactants are delivered into and start to react with an open-topped vessel from which they pass via an overflow outlet into a channel extending over the fall plate to the conveyor. As seen across the width of the channel, there is a significant variation in the speed at which the reacting foam mixture flows along the channel from the vessel outlet to the conveyor. More precisely, the speed at the centre is substantially greater than that at the lateral sides of the channel. A top sheet of paper, as conventionally used in other types of foam production plants, applied over the reacting foam mixture leaving the vessel, is unable to conform to and move with the foam over the width of the channel along which the foam and paper advance, and this has the consequence that the paper creases and creates cracks and crevices in the underlying foam. The damage thus caused to the resultant foam bun means that less usable foam is obtained compared with the Maxfoam process operated without any covering sheet applied over the foam.
The objective of the present invention was to find a solution to the problem which is outlined above and which had previously thwarted others who had addressed it in the past.